I can't say for sure. Once you add Grim to your family, he still keeps his smoke trail and voice, I believe, but he becomes a normal sim and a new Grim is generated in his place. I've added him to families multiple times, but once I added Grim to my family and edited the heck out of him and I kept finding piles of trash and the little black puddles that Grim leaves behind when he drinks anything at my the hospital with Get to work, I don't know why, but I'd assume it would have something to do with me meddling with Grim...
So you could try, but maybe back up your save just in case. I don't think I messed up the game much more than that, but the black goo was annoying, it kept generating.
